WASHINGTON — The U.S. economy grew at a 2.5 percent annual rate from April through June, an improvement from the first three months of the year. But economists are worried that growth may be slowing. The Commerce Department said this morning that its final look at economic growth in the spring was unchanged from a prior estimate made last month. However, the components of growth were altered slightly. Businesses added a bit less to their stockpiles and exports did not grow as fast as previously thought. These downward revisions were balanced by slightly stronger spending by state and local governments. Many analysts believe growth is slowing to a sluggish rate at or below 2 percent in the current quarter. Economists had initially hoped growth would improve in the second half of the year.